Quantum wells as heat detectors.

Quantum wells as heat detectors

A quantum-well heat-detection device based on a simpler gallium-arsenide/gallium-aluminum arsenide structure is described in the July 25 APPLIED PHYSICS LETTERS by Barry Levine and colleagues from AT&T Bell Laboratories in Murray Hill, N.J. Today's best infrared (heat) detectors -- largely used by the military (as night scopes to view troops and weapons) and on remote-sensing satellites -- rely on mercury-cadmium-telluride semiconductors. While very sensitive, these are also difficult to make and reproduce, notes Federico Capasso, head of quantum phenomena and device research at Bell Labs and a manager of the quantum-well heat-detector project. In contrast, he says, the AT&T heat sensor is based on a common, easy-to-produce material.

The heart of the new photodetector is a crystalline superlattice containing 50 units -- each a 40-angstrom-wide quantum well whose sides are 300-angstrom barriers of aluminum-gallium arsenide. At the bottom of each well are a multitude of electrons. As a 10-micron wavelength photon enters the well, it excites an electron -- essentially kicking it up above the barrier and outside the well. At this higher energy level, the electron is unbound and free to "sail through the whole superlattice structure," Capasso exlains. Wells are refilled from a "reservoir" of electrons slowly tunneling through the crystal. Varying the well's width and its barrier composition will allow detection of up to 5-micron photons.

Although Capasso's group has been working on the 10-micron heat detector for only about one year, he says it "already starts to match the performance of detectors that have been out there for 20 years." Moreover, because it is based on the well-established gallium-arsenide semiconductor technology, he says, it offers the promise of costing less and for the first time making a 10-micron detector and the electronics for analyzing that signal out of the same chip.